Due to increases in the speed at which digital circuits operate, clock circuits that generate reference clocks for digital circuits have to operate at higher frequencies, for example in the 100 MHz range. An artifact of higher frequency clocks circuits is that electromagnetic interference (EMI) emissions as a result of clock signal transmissions can significantly exceed the level of thermal noise. This is a problem because in some technologies receiver sensitivity is approaching the level of thermal noise. For example, the state-of-the-art receiver for receiving 802.11 encoded transmissions as defined by the Institute of Electrical and Electronics Engineer (IEEE), can work at a received power of −96 dBm, whereas the in-band thermal noise is −101 dBm. Further, the state-of-the-art Global System for Mobile Communication (GSM) receiver has a sensitivity of −110 dBm, and the in-band thermal noise is −119 dBm.
One technique to reduce the EMI emissions when transmitting a clock signal is to spread the clock frequency by modulation of the clock period. This technique is known as spread spectrum clocking. The spread is about 0.25–1.5% of the center frequency, and helps to reduce the peak of the emission at the center of frequency. However, the spread spectrum clock still emits electromagnetic interference that is higher than the background noise. For example, on a typical laptop, clock emission is 10–20 db higher than the thermal noise, and thus limits the performance of wireless technologies such as Wireless Local Area Network (WLAN), Wireless Wide Area Network (WWAN), Wireless Personal Area Network (WPAN), using a laptop.